The present invention relates to a unique safety interlock for microwave ovens. In microwave heating appliances, the nature of the heating phenomenon is that of stressing certain of the molecules of the products to be heated by using an electromagnet field, commonly in the heating frequency range of 2450 MHz. A problem with such devices has been concern about radiation leakage and the resulting possibility of serious injury to the operator. Because of this danger, the Department of Health, Education and Welfare, through the Bureau of Radiological Health (BRH), has promulgated a series of regulations specifying minimum safety precautions for microwave ovens manufactured or sold in the United States.
All such ovens are required to have a minimum of two safety interlocks. At least one safety interlock must be concealed and further, the concealed interlock is not to be operable by:
"A. ANY PART OF THE HUMAN BODY,
B. ANY OBJECT WITH A STRAIGHT INSERTABLE LENGTH OF 10 CENTIMETERS, OR
C. A TEST MAGNET HELD IN PLACE ON THE OVEN BY GRAVITY OR ITS OWN ATTRACTION." It is further required that "any visible actuating member of the concealed safety interlock must not be intended for removal by conventional tools without full or partial disassembly of the door and must have an apparent useful purpose and function other than interlock actuation unless access to the interlock is prevented when the door is open." Also it is required that a means of monitoring at least one of the safety interlocks be provided to cause the oven to become inoperable if the safety interlock should fail.
A number of interlocking devices have been developed for use with microwave ovens. Many of these devices, such as for instance those disclosed in U.S. Pat. Nos. 3,816,688; 3,715,554; 3,715,552; and 3,699,299, utilize a blade or latch member attached to the oven door for interlock actuation. The blade is positioned so as to be inserted into a slot in the oven cabinet when the door is closed. A switching mechanism within the cabinet is actuated by the presence of the blade and oven operation is thereby enabled.
Another approach taken to the interlock problem is disclosed in U.S. Pat. No. 3,784,776. The locking member disclosed therein is positioned within the cabinet of the oven and is physically moved into locking position by a solenoid. This movement of the locking member is sensed by means of a switch adjacent to the solenoid. Another solenoid actuated locking device is disclosed in U.S. Pat. No. 3,823,294. FIG. 6 of that disclosure shows an electromagnetically driven latch comprising a latching member having a hook for engaging the door and being pivoted by a solenoid. The position of the latching member is sensed by a switch.
Latches requiring large solenoids to move latching members are somewhat expensive and may require frequent service. Additionally since latching mechanisms are ordinarily susceptible to actuation when the oven door is opened, additional independent sensing means must necessarily be placed in the interlock circuitry to detect the opening of the oven door. A device for sensing the opening of the oven door is disclosed in U.S. Pat. No. 3,504,144.